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composition of soils, and what are the effects of texture, color and mois¬ 
ture on the temperature of soils and on the growth of plants. He should 
also know the form in which various substances are absorbed by plants, 
and which are the organs, and what the power by which the crude sap 
is digested and made fit to enter into the various forms of beauty and 
usefulness which exist in the vegetable kingdom. 
Plants consist of water; of organic matters, certain gases, derived 
chiefly from the decomposition of animal and vegetable substances; and 
of inorganic or mineral matter. By drying in a bath or oven heated by 
boiling water, the per-centage of water in plants is ascertained. Potatoes 
contain about 4-5ths and the turnips about 9-10ths of their weight of 
water. By burning the dried plant in the open air, the organic constit¬ 
uents are burned away. The ash which remains is the inorganic matter, 
and consists chiefly of alkaline salts. 
The organic elements of plants consists of carbon, oxygen, hydrogen, 
and nitrogen. Carbon constitutes from forty to fifty per cent, of plants 
in the dry state, commonly grown for food. It is found in plants in a 
much greater proportion than the other organic elements, and it was for¬ 
merly considered to be the chief substance concerned in the nutrition of 
plants and the main cause of the difference in the fertility of different 
soils ; latterly, however, it has been concluded that plants may obtain 
their carbon from the air, providing all other constituents required by 
them exist in the soil in an available condition. Charcoal, as is generally 
known, is an impute state of carbon ; it is insoluble in water, and can¬ 
not therefore enter the plant in that state. Carbon is obtained by plants 
chiefly in combination with oxygen, as carbonic acid gas. It is absorbed 
by leaves in the gaseous state from the atmosphere, and along with water 
by the roots. Liebig, in his chemistry of agriculture and physiology, 
stated, that “when a plant is quite mature, and when the organs by which 
it obtains food from the atmosphere are formed, the carbonic acid of the 
soil is no longer required. During the heat of summer a plant derives 
its carbon exclusively from the atmosphere.” On the first appearance of 
Liebig’s work this statement gave rise to much controversy, and many 
doubted the accuracy of his views on this point. He mentions one fact 
which is somewhat difficult to reconcile with this conclusion. When 
stating that the products of a plant may vary according to the nature of 
